The present invention relates generally to an apparatus for verifying the position of a movable member. More particularly, the present invention is directed to a control element assembly (CEA) position indicator that includes a CEA position verification apparatus.
Nuclear power plants typically include independent shut-down and safe-operations systems that monitor plant operation and evaluate numerous safety-related parameters. In the event one or more measured parameters indicate the existence of an unsafe condition, the shut-down system designed to mitigate the effects of an anticipated transient condition and/or the safe-operation can automatically effect the appropriate remedial action. It is imperative that these safety control systems, known as plant protection systems, operate reliably, and accordingly, it is imperative that all measured and sensed parameters be valid.
In the context of nuclear plant protection systems, it is not uncommon to measure a multitude of parameters related to plant operation. These parameters include, for example, temperatures, pressures, flow rates, power density, neutron flux, fluid levels, etc. Other functions of the plant protection system include the status-monitoring of various components including valves, pumps, motors, control devices and generators.
Additionally, the plant protection system, under certain defined conditions, may initiate a reactor trip (RT), i.e., the rapid, controlled, and safe shut-down of the reactor by actuating various field systems and remote actuation devices. In the case of a pressurized light water reactor, the shut-down is often accomplished by the dropping of moderating control rods into the reactor core to cause the reactor to become sub-critical.
The rod assemblies are comprised of four or twelve rods joined by a spiderlike connecting flange. Each rod assembly is commonly referred to as a control element assembly (CEA). The CEAs are arranged in groups or sub-groups consisting of a minimum of four CEAs. The connecting flange couples the CEA to a control element assembly drive mechanism that controls the movement of the CEA in and out of the reactor core. The control element assembly drive mechanism normally includes a position indicator that senses location of the CEA. As the CEA controls the overall reactor power level and provides the principal means of quickly and safely shutting down the reactor, sensing and monitoring the CEA""s position in a nuclear reactor is imperative.
There exist well known systems for sensing and monitoring CEA position. In U.S. Pat. No. 3,656,074, which is assigned to the Assignee of the present invention and incorporated herein by reference, there is described a CEA position sensing apparatus 10. As shown in FIG. 1, the apparatus includes a CEA represented by a single control rod located within a control rod housing and a control rod drive motor; a permanent magnet physically positioned on the CEA drive shaft; and a position transmitter including a plurality of flux responsive devices and a voltage divider network.
As the CEA illustrated in FIG. 1 is represented as a single control rod, only one control rod housing 12 is shown extending upwardly from the top of the reactor 11. The control rod housing 12 will typically be a nonmagnetic stainless steel tube that is approximately five inches in diameter having a one inch thick wall. The control rod drive shaft is situated within housing 12, and the control rod itself extends into the main portion of reactor 11.
A control rod drive motor 14 is mounted above a cap which seals the upper end of housing 12. Through appropriate gearing, the control rod drive motor 14 causes the control rod drive shaft to move axially relative to housing 12, thus adjusting the position of the CEA relative to the reactor core. The control rod drive shaft of motor 14 communicates with the interior of housing 12 through specially designed seals that are available commercially.
Permanent magnet 16 is mounted on the control rod drive shaft. The magnet 16 must be constructed of material capable of withstanding the highly corrosive conditions of the environment to which it will be exposed, as the magnet will be located inside a nuclear reactor. Further, the magnet 16 must be sufficiently strong so that its flux field will bridge the thick stainless steel walls of the housing 12.
As described below, a portion of the position sensing apparatus 10 is mounted on the exterior of the housing 12 and is contained within a separate transmitter housing 18. The housing 18 comprises an elongated hollow member, generally of tubular form, comprised of a nonmagnetic material which preferably has a high coefficient of thermal conductivity. The upper end of housing 18 is adapted to receive an electrical connector 20.
The connector 20 provides a means for connecting the position transmitter circuitry to external circuitry in a manner which permits removal of the connector 20 from the housing 18. The connector 20 also electrically couples the output of the position sensing apparatus 10 to a position indicator 22.
FIGS. 2(a) and 2(b) respectively show top and side views of the position transmitter circuitry located within the housing 18 through cut-away portions of said housing. Shown within the housing 18 is a terminal strip 24 which is mounted to a reed switch position transmitter 26 including a plurality of magnetic flux responsive switches and the components of an incremental potentiometer. The flux responsive devices are shown as reed switches 28 and 28xe2x80x2, and the components of the incremental potentiometer are shown as resistors 30. The reed switches 28, 28xe2x80x2 and the resistors 30, are mounted to terminal strip 24 and electrically interconnected by means of standoff and feed through connectors 32. The reed switches 28, 28xe2x80x2 are spaced on the terminal strip 24 at uniform incremental distances small enough to insure that at least one of the switches will be actuated from any location of the magnet.
In the preferred embodiment, the reed switches are wired in pairs as designated in FIG. 3 by the reference numerals 28, 28xe2x80x2. The reed switches 28, 28xe2x80x2 have a length of approximately one inch, and are disposed in serial axial alignment parallel to the path of magnet 16. The reed switches 28, 28xe2x80x2 forming the reed switch pairs are spaced apart approximately one inch to provide an arrangement wherein a small overlap of switch actuation will occur, thus reducing the possibility of a CEA position at which no switch would be actuated. As the control rod drive shaft travels axially within control rod housing 12, switches 28, 28xe2x80x2 will be sequentially closed at the approach of the field of magnet 16 and opened after the magnetic field passes. The switches are arranged such that serially adjacent switches will be closed as the magnet 16 is intermediate the two switches.
With regard to the electrical portion of the position indicating apparatus 10, FIG. 3 shows a plurality of resistors 30 of the same size and type connected at end points 33 and 34 across the power supply 38 (power supply 38 shown in FIG. 1). The resistors 30 form an incremental potentiometer or voltage divider. As discussed above, reed switches 28xe2x80x2 are electrically connected in series with each of switches 28, and are positioned in substantially the same locations as switches 28. Each of reed switches 28 is connected to a different point or tab on the voltage divider comprising resistors 30. All of the circuits comprising the series connected switches 28, 28xe2x80x2 are connected to a signal bus bar having a terminal point 20. Thus, upon closing of one of the switches 28 and its serially connected back up switch 28xe2x80x2, a signal from the incremental potentiometer comprising resistors 30 will be applied to bus bar terminal 20. The amplitude of this signal indicates which switch of switch pair 28, 28xe2x80x2 is at that instant subject to the field from magnet 16.
Another system for sensing, monitoring and transmitting an indication of control element assembly (CEA) position is described in U.S. Pat. No. 5,333,160, assigned to Assignee of the present invention and herein incorporated by reference. As shown in FIGS. 4 and 5, the apparatus 100 includes a CEA represented by a single CEA 120 located within housing 140, a plurality of magnetic circuits 102 comprising at least one magnet 104 and at least one acruate-shaped magnetic path 106, and at least one position transmitter assembly 108 including a plurality reed switches 28, 28xe2x80x2 or other magnetically responsive switches and a voltage divider network or incremental potentiometer 29.
The CEA 120, housing 140, reed switches 28 and 28xe2x80x2, and voltage divider network 29 may take the form of like elements disclosed in U.S. Pat. No. 3,656,074, the explanation of which is incorporated herein by reference. A control rod drive coil stack 142 is mounted on the control rod housing 140. Magnetic flux from the coil stack 142, acting through the stainless steel housing 140 causes the CEA 120 located inside the housing 140 to move axially, thus adjusting the position of the CEA relative to the reactor core. As the CEA 120 is withdrawn from the reactor, a control rod extension shaft 150 moves up into an extension shaft housing 190 above the CEA 120 housing.
The vertical position of the CEA 120 may be detected by determining the position of the extension shaft 150. The CEA 120 position transmitter includes a number of magnetic circuits located on the outside of the extension shaft housing 190 at a predetermined number of elevations. For example, the magnetic circuits may be spaced one inch apart over an eleven foot length of the extension shaft housing 190. Each magnetic circuit has at least one magnet 104, a carbon steel magnetic path 106 surrounding the extension shaft housing 190, and at least one longitudinally extending reed switch position assembly 108. Referring to FIG. 5, there are provided a pair of magnets 104 and a pair of position transmitter assemblies 108 to provide redundant monitoring of CEA""s 120 position for increased reliability.
At selected elevations along the direction of travel of the CEA 120, a measurement is made by providing a magnetic circuit around the extension shaft housing 190 which senses the presence of the extension shaft 150, depending on how far the control rod is withdrawn. In the preferred embodiment, the extension shaft 150 is composed of a ferromagnetic material. The strength of the magnetic field in the area of the position transmitter assembly 108 will be dependent on the reluctance of the magnetic circuit, which in turn depends on how much ferromagnetic material is present in the magnetic circuit. Since the ferromagnetic extension shaft 150 moves in the path of the magnetic field, the magnetic field strength at a particular elevation in the area of a particular reed switch 28, 28xe2x80x2 will be greater if the extension shaft 150 is at or above that elevation, and less if the extension shaft 150 is below that elevation. The sensitivity of the reed switches 28, 28xe2x80x2 is chosen so that the reed switches 28, 28xe2x80x2 will close in the stronger magnetic field and open in the lesser field.
FIGS. 2(a) and 2(b), respectively, also show top and side cut-away views of a transmitter circuitry located within the housing 190. Shown within the housing is a terminal strip 24 to which are mounted flux responsive reed switches 28 and 28xe2x80x2 and other components of an incremental potentiometer of the position transmitter 108. The reed switches 28 and 28xe2x80x2 are spaced along the terminal strip 24 at uniform incremental distances corresponding with each predetermined elevation of the magnetic circuitry.
The other components of the incremental potentiometer 29, resistors 30, are mounted to the terminal strip 24 and are electrically interconnected by means of standoff and feed through connectors 32.
Referring to FIG. 3, a schematic diagram for the electrical portion of the position transmitter assembly 108 is shown. A plurality of resistors 30 of the same size and type are connected at end points 33 and 34 across the power supply 38 to form an incremental potentiometer or voltage divider 29.
Standard reed switch position transmitter (RSPT) systems may result in an unnecessary reactor trip if there are erroneous indications of the location of the control element assembly (CEA) sent to the control element assembly calculator (CEAC). Certain malfunctions may cause CEA position locators to incorrectly report the location of the CEA. These malfunctions may consist of the failure of electrical devices or other components of the system. If any component of the system fails, an incorrect rod position will be input to the CEAC, resulting in the generation of a penalty factor, which is input to the core protection calculator. The core protection calculator uses the penalty factor information, along with other parameters to determine if safety limits are exceeded and initiates a reactor trip if limits will be exceeded. Consequently, a position locator system is needed that may reduce the likelihood of a false CEA position signal.
Accordingly, it is an object of the present invention to overcome the above-mentioned and other disadvantages of the prior art by providing a position verification apparatus that independently senses and transmits to the control element assembly calculator (CEAC) a signal indicative of the position of the CEA.
It is another object of the present invention to provide a position verification apparatus that may be easily incorporated into existing reed switch position transmitter (RSPT) systems.
It is a further object of the present invention to provide a position verification apparatus that may increase the reliability of current RSPT systems.
It is a further object of the present invention to provide a position verification apparatus that is inexpensive and easy to install on existing nuclear plants.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied, this invention may comprise a movable member disposed within and movable with respect to a housing containing the movable member, a means for generating a magnetic field within the housing, a first magnetically responsive means for sensing the position of the movable member within the housing and outputting a signal indicative of the position of the movable member, a second magnetically responsive means for sensing the presence of the movable member and outputting a signal indicative of the position of the movable member and a means for comparing the signal output from the first magnetically responsive means and the signal output from the second magnetically responsive means.
In an alternative embodiment, the components of the position transmitting apparatus, the first and second magnetically responsive means and the voltage divider network, may be contained within a transmitter housing positioned adjacent to the housing containing the movable member. The means for generating the magnetic field may include a permanent or an electrically excited magnet. The magnet generates a magnetic field proximate the movable member. Thus, the magnetic field is strongest in the housing at the point immediately adjacent the movable member and closest to the source of the magnetic field. Consequently, as the movable member travels axially within the housing, the intensity of the field strength at various points within the housing varies depending on the location of the movable member.
The first magnetically responsive means includes a plurality of switch means such as reed switches. The reed switches are electrically connected to a voltage divider network, and are uniformly spaced apart a predetermined vertical distance such that at least one switch is always energized, closed. The reed switches are electrically coupled to a connector that receives an input signal from the reed switches that is indicative of the position of the movable member.
The second magnetically responsive means also comprises a plurality of reed switches spaced apart a predetermined vertical distance. The reed switches comprising this second set of reed switches are electrically coupled to a signal contact which receives a signal from the second set of switches indicative of the position of the movable member.
When the position transmitting apparatus is installed on a nuclear plant, the movable member is replaced by a control element assembly (CEA). The control element assembly calculator (CEAC) receives the signal from the signal contact (described above) electrically associated with the first set of switches and that from the signal contact electrically associated with the second set of switches. The CEAC compares the signal from the first set of switches with that received from the second set of switches to determine the value of any penalty factors, if any, that must be generated to control the reactivity of the reactor. If the output from the first set of switches and the second set of switches is inconsistent with regard to the location of the movable member, the signals are ignored. If the output from the first set of switches indicates that the CEA is at a particular location and the output from the second set of switches confirms that the CEA is not at the top, the location of the CEA is assumed to be as indicated by the first set of switches.
Finally, the position verification apparatus includes a display for visually representing the position of the movable member.
In operation, the switches forming the first set of magnetically responsive switches sequentially open and close as the magnetic field of the magnet becomes intermediate the axial position of a particular switch. The strength of the magnetic field causes the switch to close. The switches are arranged such that at least one switch forming the plurality of switches comprising the first set of switches is always closed.
The second set of magnetically responsive switches are located primarily at a position corresponding to the CEA being withdrawn to the top of the core. At least one switch included in this second set of switches will remain closed, activated, as long as the CEA is positioned so that the CEA is outside the reactor core. Once the CEA penetrates the reactor core, at least one switch included in the second switches will remain open, indicating that the CEA is no longer positioned at the top of the core, but is positioned within the core.